1. Field of the Invention
This invention relates generally to alarm systems for facilities (or equipment) whose security is to be monitored, and more particularly to systems in which monitoring is carried out by automatic equipment that is not in the same location as the secured facility.
2. Prior Art
Conventional intrusion-alarm systems have wires that run from a power or signal source through intrusion sensors to a control unit that monitors the status of the sensors. The simplest intrusion sensors have only two states, "alarm" and "secure," indicated by a switch that is open or closed (usually respectively). The most familiar example is the magnetic switch used on doors and windows with burglar-alarm systems.
One strategy for an intruder who wishes to gain entry is to "deceive" such a system by shorting the wires--or by determining and injecting via a simple electrical splice whatever signal is required to indicate the secure status. The subject facility can then be breached without generating an "alarm" at the monitoring apparatus, even though the condition of the sensor(s) is forced into the "alarm" condition.
This strategy has its analogy for more modern systems in which the signals are optical and are carried on optic fibers: the intruder must
(1) know generally how the system works, and PA0 (2) either (a) know which fiber carries the probe signal and which the reply signal, or (b) be prepared to inject the proper optical signal into both fibers, and PA0 (3) either (a) know the necessary signal parameters, or (b) determine them by finding and forming a slight defect in the transmission characteristic along one of the fibers, then coupling optical energy out of the fiber at that point, and observing the parameters of that tapped signal, and PA0 (4) formulate or obtain a deception signal that simulates the necessary parameters, and PA0 (5) find or form a slight defect in the transmission characteristic along the reply-signal fiber, and PA0 (6) inject the deception signal via the defect into the fiber.
The equivalent of "shorting" is awkward or impossible because it is hard to construct or form an efficient energy-transmitting tap, along either the probe-signal fiber or the reply-signal fiber, without interrupting signal transmission along the fiber at the prospective tap site. Thus an alarm will be generated in the course of trying to effectuate the optical "short." This limitation, however, is not crucial to the efforts of an intruder in prior-art systems because the parameters of the signals used have been determinable by prior knowledge or observation--and in most cases have been fairly simple--and have been relatively easily to simulate. Therefore it has been unnecessary for an intruder to "short" the probe and reply signals. The intruder simply "works around" this requirement by determining and simulating the probe signal.
Because prior systems have been relatively easy to defeat in the ways just described, we have sought to provide a system that renders ineffective the intrusion strategies described above. We have invented a system which effectively precludes alternatives (2)(b) and (3)(a) of the numbered steps in the preceding description, and which makes steps (3)(b) and (6) extremely difficult--and perhaps, under the conditions in which a would-be intruder must normally work, impossible.
Moreover, even if a would-be intruder successfully surmounts the plain difficulties of steps (1), (2)(a), (3)(b), (5), and (6), our invention in its more elaborate forms renders even more difficult the performance of step (4).